1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the invention relates to a liquid crystal display device in which a thin film transistor has a high ON-current for pixel control, quick writing of video signals are achieved, and there is less variation in ON-current characteristics.
2. Description of the Related Art
A liquid crystal panel for use in a liquid crystal display device includes a TFT substrate having pixel electrodes, thin film transistors (TFT), or the like formed thereon in a matrix form, and a counter electrode opposing the TFT substrate and having color filters, or the like formed at positions corresponding to the pixel electrodes of the TFT substrate. Liquid crystals are put between the TFT substrate and the counter substrate. The liquid crystal display device controls light transmittance for each pixel by using liquid crystal molecules to form images.
The number of pixels has increased in the screen of the liquid crystal device due to enlargement in the size thereof and high definition. Since, in such a case, the frequency at which video signals are written in one frame increases, the time it takes for writing video signals to each pixel is restricted. For enabling short-time writing to each pixel, the ON-current of a TFT in the pixel needs to be increased while the OFF-current of the TFT needs to be kept small.
To address the problems as described above, JP-A-11-17188 describes a configuration in which a contact portion between a semiconductor layer formed of a-Si that operates as an active layer of a TFT and a drain electrode or a source electrode is formed by two layers of a n+-a-Si layer and a n+-poly-Si (microcrystalline silicon) layer.